1. Field of the Invention
This invention relates to containers, adapted for holding food, which are suitable for use in microwave oven cooking.
2. Description of the Prior Art
It is known in the art to produce various shaped articles from foamed and unfoamed thermoplastic materials such as polystyrene sheet by thermoforming methods. Many such articles are containers used for packaging foods.
Many thermoplastic containers, however, are melted by hot fats and cannot be used to package fat-containing foods that are to be heated in a microwave oven. In particular, polystyrene containers are not suitable for microwave cooking. Similarly, containers made from poly(p-methylstyrene) are not, per se, resistant to hot fats when heated in a microwave oven. In general, containers made from thermoplastic polymers or copolymers with a softening temperature below 190.degree. F. (87.8.degree. C.) are not resistant to hot fats when heated in a microwave oven.
Microwavable thermoplastic containers are described in a copending U.S. patent application of John P. Marano and Maya S. Farag, Ser. No. 164,993, filed July 1, 1980, now U.S. Pat. No. 4,335,181. The containers disclosed in that application are made from polymers which are not in themselves microwavable in the presence of hot fats, for example, polystyrene, poly(para-methylstyrene) and polyethylene. Melt-through in the presence of hot fat is avoided by laminating to the substrate a protective film such as polyacrylonitrile or polyethylene terephthalate.
Containers made from poly(para-methylstyrene) which are resistant to hot fats under microwave cooking conditions are described in U.S. Pat. No. 4,205,114 to Peter J. Canterino and Harold A. Arbit. Fat resistance is obtained by cross-linking the poly(para-methylstyrene) with ionizing radiation.